rose: Fix signedness issues wrt. digi count.

[ Upstream commit 9828e6e6e3f19efcb476c567b9999891d051f52f ]

Just use explicit casts, since we really can't change the
types of structures exported to userspace which have been
around for 15 years or so.

Reported-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>